Of all the volcanoes of Iceland, the most famous is Hekla. It is one of five recognized stratovolcanoes in Iceland, and of those it is by far the most active. Over the past millennium, there have been around 20 eruptions of Hekla, accounting for 13% of all Icelandic eruptions. There are more prolific eruptors on Iceland (Grimsvotn and Bardarbunga come to mind), but those volcanoes are far removed from populated areas. Hekla is near to where people live, and its eruptions have affected those people, often badly. On average, the eruptions are about a life time apart; over Iceland’s human history, most people would have experienced one of its eruptions. Once Iceland became colonised, Hekla quickly build itself a reputation, and it was a fearsome one. Across Europe, this volcano became known as the gate of hell.
In this island there is likewise a mountain whose floods of incessant fire make it look like a glowing rock and which by belching out flames keeps its crest in an everlasting blaze. This thing awakens our wonder, namely, when a land lying close to the extreme of cold can have such an abundance of matter to keep up the heat as to furnish eternal fires with unseen fuel and supply an endless provocative to feed the burning.
This was written around 1200 by a Danish historian, Glaxo Grammaticus, in the Gesta Danorum. It is a description of Hekla, and the floods of incessant fire appear to refer to its prolific lava flows. Over just the 20th century Hekla has produced 1.2 km3 of lava. But it is not a one-trick pony. I also produces tephra, amounting to 0.15 km3 (DRE) over the same time. It combines frequent medium-sized explosive and effusive eruptions. It is not a good neighbour. But is it hell on earth?
Over its history, Hekla has presented different facets: it has not always behaved in the same way as it does now. Between 7000 and 3000 years ago it had infrequent but large explosive eruptions which covered much of Iceland in tephra. It left layers in the soil which are still easily recognised. In fact they are so clear that these layers have been given names. There are four layers called Hekla‐3, ‐S, ‐4, ‐Ö, ‐Hekla-5. At this time, this was a typical rhyolite-like system. There probably was no major mountain. The Hekla-4 layer traces one of the largest explosions in Iceland over the past 10,000 years. Ten kilometers north of Hekla, this layer is still a meter thick. The explosion is estimated at VEI-5, and there are some indications that it originated from a caldera.
After the final Hekla-3 eruption, the volcano changed its behaviour. Now it create many more but thinner layers in the soil. Eruptions became much more frequent but as shown by the layers, they were smaller: either the conduit had cracked, or the magma had changed. In fact, the layers show evidence that the magma had become more basaltic. The eruptions were still explosive: it had not become a Bardarbunga. Each explosion lasted only hours, as indicated by the fact that each tephra layer was deposited in one direction only. The move to a more basaltic composition makes it likely that these explosions were followed (or interspersed with) effusive eruptions. This is the usual mode at the moment: a short explosive phase followed by effusion.
In other words, this is when the lava began to flow. It may also be when the edifice of Hekla began to grow, build up by the numerous lava flows. The Hekla we see may only be 3000 years old! Is this growth phase still on-going? That is unclear (albeit not unlikely). At the moment, Hekla is 1490 meters high, and it has had this height for the last few decades. But as recent as 1907, the height was reported as 1447 meters. Some of the difference can be due to the lava flows of the 20th century, especially the 1947 eruptions which formed layers of 10 meters thick or more, partly around the summit. But Hekla may have had ups and downs. Earlier in the 19th century the mountain was reported to be 1489 meters high, the same as today. This measurement is probably not very accurate, though. But even if the height is not varying, the bulk is probably increasing because of the high lava production.
The new eruption mode of smaller explosive/effusive eruptions has continued over the past 3000 years. But these eruptions were not always equally frequent. The Vikings arrived at a time when Hekla’s eruptions had grown infrequent. For the first 200 years of their presence, Hekla slept: a dormant and unremarkable mountain. A fertile valley north of Hekla, Þjórsárdalur, became home to a farming community. Then, suddenly, Hekla woke up. The explosion that followed was the second largest in Iceland since the settlement (only the 1362 explosion at Öræfajökull was larger). Soon after, Hekla had become a world renowned volcano rivalling Etna. And those farms lay in ruins.
Þjórsárdalur is a deep valley on the edge of the Central Highlands, some 15 kilometers north of Hekla. Nowadays it is known for some spectacular waterfalls, but it may have looked different 1000 years ago – Hekla has not been kind to this area. The fertile soil made it ideal for farming, and a small settlement was soon established. We currently know of 21 separate farms that existed in the area. They probably were not all active at the same time, and some of the farms may never have been viable because the soil eroded too easily under use. But this was a prosperous area. One of the farms here, Stöng, has been rebuild as a show home (Þjóðveldisbærinn), although not in its original (fairly inaccessible) location. The ruins are located in a forested gorge called Gjáin. It is a multi-layered site: underneath the large farm were the ruins of an older farm house. The farm was started by a Viking of legend, Gaukur Trandilsson, in the 10th century. An old story related that Trandilsson became involved with the wife of a nearby farm, which led to an axe duel (ouch) and his demise. Such was Viking life. Volcanoes were nothing to worry about compared to the neighbours. Social distancing was a survival skill.
The 15th century Biskupa annalar tells that 11 farms in the Þjórsárdalur valley were destroyed by the first Hekla eruption. Stöng may have survived as a going concern, although excavations have shown that it was covered in 50 centimeter thick, white tephra: the rains kept bringing more down from the slopes to the bottom of the valley, and this depth may have build up over many years after the eruption. The valley was devastated and never really recovered. Later eruptions added more misery, and eventually this community succumbed to Hekla. Sometimes neighbours are welcome, and sometimes they make life hell. Of course, one may wonder how much of the original fertility of the valley was due to frequent dustings of volcanic ash. It is now a mute point. You can have too much of a good thing, even hell. But although this valley partly recovered after the first eruptions, farms in valleys further to the north did not. Several farms 50 to 70 km north of Hekla were abandoned at this time, and their areas were never resettled. The thick tephra made any farming impossible for many years after. And of course, the worsening climate after 1200 would not have helped in those marginal areas. Þjórsárdalur was not marginal, though – it was just too close to hell.
The first recorded eruption of Hekla became famous across Europe. The Vikings had experienced earlier eruptions of other volcanoes in Iceland, including rather significant ones. Eldgja coincided – presumably not accidentally – with the end of the age of settlement. But in general, the annals with the oldest stories of Iceland are silent on eruptions. They tell of the life and misdemeanors of the people in great detail, but volcanoes play little or no role in the stories. Even Eldgja, which must have devastated entire districts (and perhaps killed a large fraction of the population, seeing it was bigger than Laki), is absent from the ancient stories. (There are suggestions that Ragnarok is an echo of Eldgja.) But Hekla is different. It was so important that in the Icelandic writings, the eruptions of Hekla became numbered. The numbering is not given with absolute certainty. For instance, when the Biskupa analar (mentioned above) talks about the 1436 eruption, it states: The twenty second bishop was Gottsvin [..] in his days fire came up for the 8th time in Hekla – some say for the 7th time – and in that fire 18 farms were destroyed in one morning. The Biskup analar becomes more authorative when it talks about later eruptions (e.g. 1510) because it was written near that time and by then it is based on personal recollections. Still, for the earliest eruptions (before a hiatus in the 14th century, perhaps black-death related) there is a complete list, with surprising accuracy. For instance, the 3rd eruption of Hekla (a small one) is known to have started on 4 December 1206, based on a near-contemporary report, and the second eruption (which formed the lava field Efrahvolshraun, west of Hekla) is dated (with slightly less certainty) to 19 January 1158.
The first eruption of Hekla, the one which gave it its fame, is dated to the year 1104. How do we know that so well? Do we?
The oldest report of this eruption is in the Hungrvaka, a history of early christianity in the region. It was probably written in the early 1200’s. The author is not known but the book says that it reports ‘what I heard this man of knowledge Gizur Hallson say’. ‘This man’ is indeed well-known: he was the Lawspeaker of the Althing (parliament) from 1181 to 1202 and died as an 80-year old in 1206. The Hungrvaka tells that ‘During the episcopate of Bishop Gizur were many great events: the death of King Knut on Fyn [..] the coming up of fire in Hekla, and many other great events’. The eruption was seen as one of the defining events of the age. Gizur is not the same as the person mentioned above, by the way. He was bishop from 1082 to 1118. And ‘King Knut’ is not the well-know Canute who turned the tide (an apocryphal story) but one of his successors, King Canute IV, later known as Canute the Holy, who was king of Denmark from 1080 to his murder in Odense in 1086. The Hungrvaka is considered reliable, but it only gives the date of Hekla’s first eruption to within a few decades. It does however indicate that this was the most important natural event of that period.
Another source of information on Hekla comes from the Icelandic annals. These are chronological compilations of events in Iceland, mainly covering the 13th to the 15th century. There are several different collections, compiled by different people. They were probably written at monasteries and chapter houses. The oldest begin with the birth of Christ and list the main events of the first millennium before moving on to Iceland. The events are apparently take from yearbooks which were kept in various places. However, the emphasis is on the events rather than the dates. As they were written down much later than the events they describe, the material from the yearbooks can have become supplemented by later traditions. The tradition of keeping annals seems to have come from the south of Iceland. The proximity to Hekla makes it more likely that the information regarding Hekla’s eruptions is accurate.
The first eruption of Hekla is mentioned in four of the annals. The oldest two are the Annales regii and the Lögmanns annáll; the Gottskálksannáll and Oddaverjaannáll were compiled later, after 1500. The oldest annals are attributed to the poet Sturla Þórðarson (1214–1284).
Three of these four annals mention ‘the first coming of fire in Mount Hekla’. This phrasing already show that they were written much later, after Hekla had erupted again. Two mention a ‘sandfall winter’ connected to this eruption: this happens when the wind blows up the smallest tephra and creates sand storms. The year is given as 1104 in three, and as 1106 in the fourth source. However, the fourth source has the eruption in the same year as the establishment of the metropolitan see in Lund, and this happened in 1104 with the appointment of archbishop Asger (1104–37). This is the reason that the eruption is generally considered as having happened in 1104. But it is not considered as conclusive evidence. The four sources use very similar language, and appear to come from a single original source. The date of the bishopric of Lund is well established, but over the years it is possible that the fact that they occurred close in time became interpreted as happening in the same year.
Why did this eruption become so famous in Europe? This has a connection to this bishopric of Lund. The oldest manuscript describing the violence of Hekla, although without mentioning it by name, is De Inferno Hyslandia, part of the Liber Miaculorum. It was written by a monk called Herbert, in the monastery of Clairvaux around 1180. The document mergers several different but unnamed eruptions into a single, extended description. His information came from Eskil, who visited Clairvaux regularly. Eskil was the second archbishop of Lund, from 1138 to 1177. Eskil was responsible for Iceland until 1152 and he had close connections there; Herbert wrote a biography of Eskil based on their personal friendship. It is therefore likely that his knowledge of Iceland came second hand from Eskil. Perhaps the fact that Icelandic sources linked Hekla to the establishment of the Lund see was because of the role of the later archbishop Eskil.
What do we know about the accuracy of the Viking records? This can be illustrated with an example not from Iceland, but from Norway. A record describes events in 1263 in Orkney: The eve of St Olaf [29 July] was on a Sunday, when King Hacon [Haakon IV of Norway] lay in Rognvaldsvoe, a great darkness overtook the Sun, so that a little ring was bright around it on the outside, and that lasted a while of the day. On St Lawrence Day [10 August] King Hacon sailed out of Rognvaldsvoe over the Pentland firth. This was to battle the King of Scotland. Things went badly, he lost much of his fleet in a storm on 30 September, lost the battle, and died in December after returning to Orkney. The eclipse turned out to be a bad omen, a sign from hell. The story is clear, and calculations show that there was indeed an annular eclipse (where the moon appears slightly too small to cover the Sun) on 5 August of that year. However, one detail does not fit. The eclipse was only partial in Orkney. The annular eclipse was seen not in Orkney but in the Norwegian capital at Trondheim. The chronicle merged two separate events which both made deep impressions, into a single story. We should be cautious when uncritically using the oldest stories, written down at a later time.
Do we have any other information? Indeed, we do. Ejecta from the eruption reached Greenland, and became embedded in the ice. The Greenland ice cores show volcanic layers, and one of these layers has been identified as the Hekla 1104 eruption. In fact there is a bit of a circularity, as the fact that this date was well known may have been used in dating the ice cores. Recently, the Greenland ice cores have been re-dated using correlations with tree rings, specifically by using two strong beryllium spikes seen in both the ice and the trees. This has shifted much of the older ice records by several years, and for instance re-dated the Eldgja eruption from 934 to 939. And it also changed the Hekla date: according to this new chronology the ejecta first reached the ice in the year 1108 or 1109. It is possible that the annals were not fully correct: the eruption happened shortly after the establishment of the Lund see, but not in the same year.
The precise moment of the Hekla 1104 eruption remains uncertain, in spite of the precise year attached to the name. (The time of the year is equally uncertain, although the mention of a ‘sand winter’ suggest it may have been in the autumn.) But if nothing else, the story of Hekla’s first eruption shows how well connected Europe was in the 12th century! An unexpected eruption in faraway Iceland was noted by the archbishop in Lund, passed on to his successor, brought to Clairvaux and written up there. But those writings were never meant to be just history. This was ecclesiastic literature, and the moral interpretations were as important as the events themselves. And so the story of Hekla as a hell on earth grew. It became the place where the souls of the damned were kept, including Judas himself. The link with Lund was how Hekla obtained its reputation and where Hekla became hell.
The 1104 (or 1108) eruption itself has been studied extensively by Sigurdur Thorasinson, the giant of 20th century Icelandic volcanology who initiated the study of tephra layers. Much of this post has been taken from his book on the eruption of Hekla 1947-1948, published in 1967. Thorasinson writes that this book grew out of 30 years of field research. He mapped all the known (and unknown) tephra layers of Hekla and showed that many were deposited in a single direction only: this shows that the explosions were brief, without a change in wind direction. He noted that Hekla tephra is white, while Katla’s layers were black. And he uncovered the ancient writings about the oldest Hekla eruptions.
One of his many results is the map showing the distribution of the 1104 tephra. It shows that the wind was southerly at the time of the explosion. The area immediately north of Hekla was worst affected with 20 cm of tephra quite widespread. It was bad luck that this included some prime, albeit fragile, farmland. But a lot of the tephra also ended up on the central highlands where it could do little damage.
The white tephra has a silicate content of over 66%. That is very high for Hekla, and it suggests that the magma chamber had been sitting still for quite some time. It agrees with centuries of quiescence before the eruption. For comparison, the highest content since that time was 62% both in 1510 (after a dormant period of 120 years) and in 1947, after 100 years of solitude. Hell needs time to brood.
The volume of the tephra from 1104 is impressive. The 10-cm contour encloses 2000 km2! The total volume is estimated at 2.5 km3. It is less in rock volume, of course, as tephra is highly fragmented: the dense rock equivalent (DRE) is around 1 km3. This was a plinian eruption, not huge, but severe and damaging. The output is of course dwarfed by that of Laki. But that is the difference between explosive and effusive eruptions. Lava is like oil: if there is a hole in the right place, it will come out. Tephra is like coal: you have to use explosives to get it out and it is a dirty job.
Iceland lacks the largest explosions volcanoes can do: there isn’t enough stress in the rocks to keep the magma locked up for long enough. But Hekla is bit off-rift itself, albeit on a small side rift. It can manage to keep its magma down – sometimes. When it does, the magma evolves to become more rhyolitic (rhyolite-dacite in the case of Hekla 1104), and becomes explosive. 95% of the intermediate magma produced in Iceland comes from Hekla.
As mentioned before, Hekla is a mixed-type volcano. It can have both explosive and effusive eruptions. The 1104 eruption was explosive. Recent results suggest that it was a short-lived (hours?) but steady eruption, which as usual waned towards the end. There is no evidence that any lava was ejected. But later eruptions did produce large lava flows. Hekla is the only volcano on the main land that has these mixed eruptions in historical times. This suggest that the 1104 eruption was a throat clearing event. After that, the conduit was largely open, and eruptions became easier and occurred more often.
There is one point left to make. Hekla’s eruptions are hard to predict, apart from that longer phases of quiescence make explosive events more likely. But explosive phases tend to be short lived. The impact of these depends strongly on the direction of the wind. An easterly wind will blow the ash to populated areas. A southerly wind will deposit it in the empty highlands. Whether it is heaven or hell solely depends on how the wind blows.
And so Hekla became synonymous with the gates of hell. But this reputation may be overblown. Yes, it is a bad and anti-social neighbour who makes nearby living impossible. Some social distancing is strongly recommended. But the eruption which led to this reputation was not the worst it can do. It damaged a number of farms, and some never recovered, however the main populated areas were probably much less affected. The reputation came in part from the unexpectedness of the eruption. This was the shock of a mini-Pinatubo.
We can deal with expected disasters. It is those that surprise us that leave a lasting memory and in our minds, become hell.
Albert, March 2020